Porous Aromatic Frameworks Impregnated with Lithiated Fullerenes

Article pubs.acs.org/JPCC

Porous Aromatic Frameworks Impregnated with Lithiated Fullerenes for Natural Gas Purification Afsana Ahmed,†,‡ Ravichandar Babarao,‡ Runhong Huang,§ Nikhil Medhekar,§ B. D. Todd,† Matthew R. Hill,‡ and Aaron W. Thornton*,‡ †

Department of Mathematics, School of Science, Faculty of Science, Engineering and Technology and Centre for Molecular Simulation, Swinburne University of Technology, Melbourne, VIC 3122, Australia ‡ CSIRO Manufacturing Flagship, Private Bag 10, Clayton South MDC, VIC 3169, Australia § Department of Materials Engineering, Monash University, Clayton, VIC 3168, Australia S Supporting Information *

ABSTRACT: Natural gas, a lower emission alternative than its fossil fuel counterparts, requires the removal of carbon dioxide, known as “sweetening”, prior to its use. In this study we computationally explore the separation of methane and carbon dioxide using a new adsorbent consisting of lithiumdecorated fullerenes (Li6C60) impregnated within a series of porous aromatic frameworks (PAFs) of various pore sizes. The strong affinity of CO2 with the impregnated frameworks, confirmed by density functional theory, leads to selective adsorption over CH4. The impregnation can also double the CO2 adsorption capacity compared to the bare PAF and increase selectivity of CO2/CH4 up to 48 for an optimum amount of Li6C60, which is above the current industry benchmark. Overall, the study reveals physical insights and proposes impregnated PAFs to be promising candidates for CO2/ CH4 separations for natural gas purification.

I. INTRODUCTION Natural gas as a vehicular fuel has a number of advantages both economically and environmentally. Compared to other fossil fuels like gasoline or diesel, natural gas reduces the amount of byproduct of CO, CO2, and SO2 by 97, 24, and 90%, respectively.1,2 Natural gas contains a variable amount of methane (CH4) ranging from (27−95%), with a wide range of other components including CO2 depending on the source;3 see the world reservoir Table S1 (Supporting Information). The presence of CO2 reduces the energy content of natural gas, contributes to climate change and often leads to pipeline corrosion.1,4 To prevent this and also to increase the commercial value of natural gas, it should meet established purity specifications that are known as “pipeline-qualitymethane”. To meet this criteria, the maximum amount of CO2 concentration cannot exceed 2%.1 In addition, when natural gas is transported over great distances, the use of pipelines is too expensive and inefficient, and therefore liquefied natural gas (LNG) is a more efficient form of transport.5 To make LNG, the gas is cooled to cryogenic conditions. During this process, the CO2 present can freeze and block pipeline systems which will cause transportation issues.6 In locations such as Germany (Central European Pannonian basin) or South Australia (Cooper Eromanga basin)7 this CO2 contamination exceeds 10%. As a result, it is critical to remove © 2015 American Chemical Society

CO2 from natural gas for economic, operational, safety, and environmental reasons.8,9 For the separation of CO2 from a CO2/CH4 mixture, various technologies are available, such as chemical absorption,10,11 thermal and pressure swing adsorption,12,13 cryogenic distillation,10 and membrane separation.14,15 Among these gas separation techniques, adsorption-based separation has become a major gas separation tool in industry due to its inherent simplicity, ease of control, and relatively low operating costs.3,16 However, with the growing global demand for natural gas, separations must become more efficient for natural gas to remain economically competitive above other harmful fuel alternatives. Several families of microporous materials have been considered for the selective adsorption of CO2/CH4 mixtures such as zeolites, metal−organic frameworks, activated carbons, silica, nanotubes, and other inorganic structures.4,9,17−21 In industry, for example, the adsorption process has been successfully implemented across the USA for the recovery of CH4 from landfill gases using zeolites.3,22 To further improve the performance of these systems, adsorbents must adsorb Received: February 4, 2015 Revised: April 1, 2015 Published: April 2, 2015 9347

DOI: 10.1021/acs.jpcc.5b01144 J. Phys. Chem. C 2015, 119, 9347−9354

Article

The Journal of Physical Chemistry C

Charges were assigned to each atom within the nLi6C60@ PAF structures using density functional theory (DFT); see Figure S1 in the Supporting Information. We followed the calculation details of Babarao et al.33 DFT was implemented in DMol3 on the basis of fragmental clusters.34 The cleaved bonds of the cluster model were saturated by methyl groups to maintain original hybridization. Double-ξ numerical polarization (DNP) with the PW91 functional set was used in the DFT calculations. The basic principal of the DNP set is that it incorporates p-type polarization into hydrogen atoms and dtype polarization into heavier carbon atoms. The atomic charges calculated from DFT calculations were evaluated by fitting to the electrostatic potential function using the Merz− Kollan (MK) scheme.35,36 The prediction of CO2 and CH4 uptake inside the nLi6C60@ PAF structures were calculated by the Grand Canonical Monte Carlo (GCMC) routine. GCMC has been used widely for the simulated separation of CO2/CH4 mixtures.17,21,37−40 CO2 was represented as a three-site rigid molecule, and its intrinsic quadrupole moment was described by a partial charge model. The partial charges on C and O atoms were qc = 0.576e and q0 = −0.288e, respectively. The CO2−CO2 intermolecular interactions were modeled as a combination of Lennard-Jones (LJ) and Coulombic potentials,

more gas at higher selectivities while retaining chemical, physical, and thermal stability. Porous aromatic frameworks (PAFs) were reported as a new family of ultraporous materials with surface areas above 5000 m2/g, 5 times above that for zeolites, and thus capable of adsorbing copious amounts of gas. To date, most studies of adsorption in PAFs have focused on gas storage applications, and it is known that capacities can be drastically enhanced when the PAF surface is chemically functionalized.23−26 We recently considered PAFs for enhancing volumetric hydrogen storage at low pressure.27 Our work showed that the incorporation of lithiated fullerenes (Li6C60) in PAFs can enhance the volumetric capacity of H2 from 12 to 44 g/L. Peng et al.28 have used C60 intercalated graphite for purification of CO2 from CH4 and calculated a selectivity of 8. We expect that the metal sites on Li6C60 will increase the polarization of CO2 molecules and consquently achieve a higher CO 2 /CH 4 selectivity than bare C60. Here we considered PAFs impregnated with Li6C60 by enhancing volumetric surface area and tuning the porosity for the separation of CO2 over CH4, shown schematically in Figure 1. In this work, single component adsorption of CH4 and CO2

⎡⎛ ⎞12 ⎛ ⎞6 ⎤ qiqj σij σij Uij(r ) = 4εij⎢⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥⎥ + r ⎝ rij ⎠ ⎦ 4πε0r ⎣⎝ ij ⎠

(1)

#tab;where r is the distance between two atoms, σij and εij are collision diameter and potential well depth, respectively, qi and qj are partial charges located at site i and j, respectively, and ε0 = 8.8542 × 10−12 C/(N/m2) is the permittivity of a vacuum. The long-range electrostatic interactions were handled via the Ewald summation method. CH4 was represented by a united-atom model41 with the LJ potential parameters.42 Lorentz−Berthelot mixing rules43 were applied to calculate the interaction between PAFs, lithium, fullerene, CO2 and CH4. The force fields adopted were used previously and have been compared with the experimental data, listed in Table 1.44−46

Figure 1. Schematic of CO2/CH4 separation in Li6C60 impregnated PAF.

Table 1. Lennard-Jones and and Coulombic Parametersa LJ and Coulombic potential

within Li6C60 impregnated PAFs of various pore sizes at close to ambient conditions has been simulated. From the single component isotherms, the adsorption behavior of binary mixtures using the ideal adsorbed solution theory (IAST) was obtained.29,30 Although this material may not be economically feasible, the scientific principles underlying the performance enhancement is of value. Structure−property relationships revealed the dominant structural characteristics responsible for enhanced separation. Finally, performances were benchmarked with conventional adsorbents.

species CO2 CH4 CH4−C60 CO2−C60

σ (Å)

ε/kB (K)

C O C CCH4−C60

2.789 3.011 3.73 3.5805

29.66 82.96 148 70.5

q(e)

ref

+0.576 −0.288 0

51 51 52

CCO2−C60

3.11

32.33

OCO2−C60

3.16

54.6

CCH4−C2P

3.36

225.44

CCH4−C3P

3.90

80.52

CCH4−HP

2.75

24.16

CO2−PAF

CCO2−C2P

3.8

69.44

54

Li-CH4 Li-CO2

Li−CH4 Li−CCO2

2.89 2.4865

59.88 19.32

55

Li−OCO2

2.5975

32.31

CH4−PAF

II. MODELS AND SIMULATION In this work the PAF structures were constructed following details outlined by Lan et al.31 The structures include PAF-30X (X = 1−4), where 3 means 3D structure and X denotes the number of phenyl rings used to replace the C−C bond. Each unit cell was constructed using the Forcite module of the Material Studio package with cubic periodic boundaries.32 Lithiated fullerenes were randomly inserted within the PAF unit cell followed by geometry optimization.

site

45 53 see Supporting Information

a

Here, C3P and C2P represent sp3 and sp2 carbon atoms in all PAFs, respectively. 9348


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The Journal of Physical Chemistry C DFT calculations were performed with the Vienna Ab Initio simulation package (VASP)47 to calculate the CO2−PAF and CO2−Li6C60 binding energies. The projector augmented wave (PAW) methods48 were used to describe the core and valence electrons. The Perdew−Burke−Ernzerhof (PBE)49 was used to describe the electron exchange and correlations. The DFT-D2 method50 was applied for the long-term van der Waals dispersion corrections. The Brillouin zone was sampled centered γ centered k-point mesh. The binding energy was calculated according to the formula: E binding = Esubstrate + gas − Esubstrate − Egas

(2)

GCMC simulations were carried out for the adsorption of single components within the range of frameworks. As a widely used technique to simulate adsorption, GCMC allows the comparison of adsorbate chemical potentials in both adsorbed and bulk phases. In this method, the adsorbent structures were treated as a rigid body. A total of 107 trial moves were used for equilibration and another 107 moves for the production steps to calculate the average amount of adsorbed gas molecules. To verify the force field, comparisons were made to experimental single component CO2 isotherms24 (Figure 2). IAST has proven an effective method for predicting gas mixtures within a wide variety of porous materials, such as zeolites,17,56 MOFs,9,57 and ZIFs.21,38,58 The method considers the spreading pressure of each gas upon a uniform surface. Perez-Carbajo et al.59 recently demonstrated that IAST could reasonably describe the mixed adsorption of a five component mixture (CO2, CH4, CO, N2, and H2) within a range of zeolites (FAU, MFI, MOR, and DDR) up to 100 bar. Here the method was utilized to calculate the selectivity across a range of feed composition ratios.

III. RESULTS AND DISCUSSION The effect of Li6C60 impregnation upon adsorption up to 2 bar and 298 K is shown in Figure 2a−c for CO2 and for CH4 in Figure 3a−c for PAF-302, PAF-303, and PAF-304, respectively. The trends are highlighted with arrows. CO2 uptake reaches a maximum at a particular amount of Li6C60 whereas the CH4 uptake continually decreases with impregnation. The maximum uptake of CO2 at 2 bar was found to be 15.6 mmol/g in PAF-302 impregnated with one Li6C60 molecule, which is approximately a 100% increase in adsorption capacity compared to that of the bare PAF. The maximum CO2 uptakes for PAF-303 and PAF-304 were 12 and 11.5 mmol/g with 10 Li6C60 and 27 Li6C60, respectively, which are approximately a 98% and 47% increases compared to uptakes of bare PAF-303 and PAF-304. Moreover, the maximum numbers of impregnated Li6C60 that can fit within the PAF unit cells are 6, 17, and 40 for PAF-302, PAF-303, and PAF-304, respectively. There are two reasons for the maximum CO2 uptake at a particular loading. One reason is that the highest N2 accessible volumetric surface area (m2/cm3) was reached at that particular loading, shown in Figure 4. The second reason is that the interactions between CO2 and Li6C60 are strong whereas for CH4 these interactions are negligible, shown in Figure 5. Therefore, the CO2 uptake increases with the increased surface area whereas CH4 uptake does not benefit from the increased surface area and rather is inhibited by the loss in pore volume. The volumetric surface area was found to correlate with the CO2 uptake (Figure 4). Upon impregnation of Li6C60 in all three PAFs, the surface area and the CO2 uptake both increase up to a maximum level followed by a decrease with further

Figure 2. CO2 uptake at 2 bar and 298 K for Li6C60 impregnated (a) PAF-302, (b) PAF-303, and (c) PAF-304. The red dotted line is the experimental results of CO2 uptake in bare PAF-302 from Konstas et al.24 Arrows emphasize the trends with the increasing amount of impregnation.

Li6C60 loading. The maximum volumetric surface areas of 2096, 2140, and 2109 m2/cm3 were achieved for PAF-302, PAF-303, and PAF-304, respectively, with corresponding numbers of Li6C60 molecules of 1, 10, and 27. Therefore, the maximum CO2 uptake is a result of maximizing the volumetric surface area with impregnation. Frost et al.60 also observed structure− 9349


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Figure 4. Structure property relationships among CO2 uptake, Li6C60 loading and volumetric surface area at 2 bar and 298 K. Solid symbols represent CO2 uptake in PAFs, and open symbols represent the volumetric surface area of corresponding PAFs.

gas−framework interaction strength. The qst in bare PAF-303 and PAF-304 for CO2 is in the ranges 16−18 and 17−19 kJ/ mol, respectively, whereas, for 10 Li6C60 PAF-303 and 27 Li6C60 PAF-304 this value reaches maximum ranges of 39−41 and 41−43 kJ/mol, respectively. These values are in agreement with DFT-based calculations that predict a binding energy of 19 and 45 kJ/mol for CO2 on PAF and Li6C60, respectively, Figure 5b. These qst values of CO2 in impregnated PAFs are much higher than CH4 qst values, which are 17 and 18 kJ/mol for 10Li6C60 PAF-303 and 27Li6C60 PAF-304, respectively. These high differences in qst values also indicate the promise for high selectivity of CO2 over CH4. The radial distribution function (RDF) between Li6C60 in PAF-304 and the guest molecules is shown in Figure S2 of Supporting Information. A peak in g(r) for CO2 is observed at r = 4 Å, indicating a high density of CO2 close to charged Li6C60, whereas no significant peaks exist for CH4, indicating bulk-like gas behavior. This confirms that CO2 interacts strongly with the Li6C60 surface within the PAF, forming an adsorbed layer, whereas CH4 molecules interact weakly. Considering this, an adsorbed layer that is denser than the bulk gas phase will enhance the overall uptake and will correlate directly with the available surface area. For CH4, on the contrary, there is only bulk gas phase present, which correlates with the accessible pore volume that continually decreases with impregnation. The adsorption separation factor is defined by Si/j = (xi/ xj)(yj/yi), where xi and yi are the mole fraction of component i in the adsorbed phase and the bulk feed, respectively. For ideal selectivity yj/yi = 1, and therefore, Si/j = xi/xj. Here, the ideal gas selectivity for CO2/CH4 was plotted against CO2 uptake in Figure 6a−c and Figure S2 (in Supporting Information) for pressures up to 2 bar. This can be defined as a trade-off plot where a maximum selectivity along with a maximum uptake is desired. The highest selectivities were observed for 27 Li6C60 PAF-304, 10 Li6C60 PAF-303, and 1 Li6C60 PAF-302. Both selectivity and uptake increased up to an optimum number of Li6C60 loading and then decreased with further impregnation. For industrial application, the mixed selectivity is of interest. Here we predict the mixed selectivity using IAST for different ratios of CO2:CH4 (Figure 7 and Figure S3, Supporting Information). The feed composition is assumed to be 20:80. From the IAST predictions we observe a reduced CO2 uptake and an increased selectivity. This is because the CO2 will

Figure 3. CH4 uptake at 2 bar and 298 K for Li6C60 impregnated (a) PAF-302, (b) PAF-303, and (c) PAF-304. Arrows emphasize the trends with the increasing amount of impregnation.

property relationships that were pressure-dependent. These trends are confirmed here within the medium to low range of pressures where impregnation and ligand extension allows the control of both the surface area and pore volume. The isosteric heat of adsorption (qst) for CO2 and CH4 is shown in Figure 5a,c, respectively, and is directly related to the 9350


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Figure 5. (a) CO2 isosteric heats of adsorption with DFT-based binding energy values (solid black symbols). (b) Strongest binding sites for CO2 on PAF and Li6C60 from DFT calculations with binding energies. (c) Isosteric heats of adsorption of CH4 in bare PAFs and impregnated PAFs.

dominate available adsorption sites over CH4 but at a loss of capacity. For a higher selectivity, fewer separation stages are required and for higher CO2 uptake less material is required. For all gas mixture ratios, the selectivity of the impregnated PAFs followed the following order 27 Li6C60 PAF-304 > 10 Li6C60 PAF-303. The IAST mixture adsorption also indicated that Li6C60 impregnation within PAFs enhanced the separation efficiency of an adsorbed based system. The separation of CO2 from the 20:80 CO2:CH4 mixture is 47 and 31 for 27 Li6C60 PAF-304 and 10 Li6C60 PAF-303, respectively, which are larger

Figure 6. Ideal selectivity CO2/CH4 vs CO2 uptake at 298 K in Li6C60 impregnated (a) PAF-302, (b) PAF-303, and (c) PAF-304. Arrows emphasize the trends with the increasing amount of impregnation.

than other promising adsorbents such as covalent organic frameworks, zeolitic imidazolate frameworks and IRMOF1.61,62 The selectivities are also above the zeolite 5A currently used within industry.63 9351


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correlated with the size of the gas molecules. This may not be the case for adsorbents.

IV. CONCLUSION The adsorption of CO2/CH4 mixtures has been investigated using molecular simulation for Li6C60 impregnated PAF structures. The simulation isotherms for pure components of CO2 and CH4 are in good agreement with the literature.24 The highest adsorption selectivity for CO2 over CH4 is predicted as 47−48 for 27 Li6C60 PAF-304 and 30−31 for 10 Li6C60 PAF303. The highest volumetric surface area correlated with the highest CO2/CH4 selectivity and the highest CO2 uptake at the optimal Li6C60 loading. The results show that the available surface area within 27 Li6C60 PAF-304 and 10 Li6C60 PAF-303 offer stronger adsorption sites for CO2 than for CH4. The high CO2 adsorption selectivity at 2 bar suggests that Li6C60@ PAF could be successfully used for natural gas (CH4) purification. In comparison with other adsorbents, the impregnated PAFs showed moderate selectivities with relatively high working capacities at standard operating conditions cycling between 1 and 10 bar. There is an apparent upper bound trade-off between selectivity and capacity that the impregnated PAFs cannot overcome. Overall, the impregnated PAFs have tunable surface areas and porosity to customize separation requirements.

Figure 7. Selectivity vs CO2 uptake at 298 K in 27 Li6C60 PAF-304 and 10 Li6C60 PAF-303 at various pressures. The dashed line is selectivity value for the most commercially used zeolite 5A.63

Finally, comparisons with simulated performances for other materials64 were made (Figure 8). Here, the ideal selectivity

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 shows the composition of natural gas reservoirs across the world. Figure S1 shows the charge assignment within the frameworks for (a) PAF-302 (four types of carbon and one type of hydrogen atom), (b) PAF-303 (eight types of carbon and two types of hydrogen atom), (c) PAF-304 (eight types of carbon and two types of hydrogen atom), and (d) Li6C60 (three types of carbon and one type of lithium atom). Figure S2 shows the radial distribution function g(r) between the Li6C60 and the center of mass for CO2 and CH4 molecules. Figure S3 shows the ideal selectivity of CO2/CH4 at 298 K and 2 bar. Figure S4 shows the selectivity vs CO2 uptake at 298 K in (a) 27 Li6C60 PAF-304 and (b) 10 Li6C60 PAF-303 for different ratios of CO2:CH4. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Adsorption selectivities vs working capacity (adsorption cycle between 1 and 1 bar) for CO2/CH4 mixtures at 300 K in a variety of MOFs, zeolite, and PAFs structures. Dashed line highlights the upper bound trade-off trend.

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AUTHOR INFORMATION

Corresponding Author

*A. W. Thornton. Tel: +61418438423. E-mail: aaron. [email protected].

and the working capacity (or delta loading) were used. The working capacity is defined as the difference between uptakes at desorption (1 bar) and adsorption (10 bar) pressures. Twentyseven Li6C60 PAF-304 and 10 Li6C60 PAF-303 showed moderate selectivities with relatively high working capacities compared to those of the other adsorbents. Higher selectivities were found at 2 bar, which means that the separation could be performed at lower pressures, assuming that the materials is completed evacuated of gas. This could possible reduce the energy requirements for separation. There is an apparent upper bound trend observed for all the adsorbents, as highlighted by a dashed line in Figure 8. For our materials, an increase in selectivity is associated with a loss in working capacity. Within the membrane literature this has been

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.A. acknowledges the top-up scholarship provided by the CSIRO Computational and Simulation Sciences Transformational Capability Platform and the full scholarship provided by Swinburne University. The authors acknowledge the computational facilities and services provided through the CSIRO Advanced Scientific Computing, Monash Sun Grid, and the National Computing Infrastructure facilities. M.R.H. acknowledges the ARC for support (FT130100345). 9352


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